1. Field of the Invention
The present invention relates to the field of optical communications and, more particularly, the invention relates to devices for selective routing of components of wavelength division multiplexed (WDM) optical signals. Specifically, the invention concerns devices adapted selectively to route components of WDM optical signals.
2. Technical Background
Wavelength division multiplexing (WDM) is increasingly being used in optical communications networks and the like in order to multiply the number of channels that can be transported along the optical fiber or waveguide. Recent demands for even greater multiplication of channels have led to the development of so-called xe2x80x9cdensexe2x80x9d WDM systems (DWDM). In such WDM and DWDM systems, it is necessary to be able selectively to withdraw from, or inject into, the main fiber or waveguide signals at particular wavelengths; this is generally referred to as the xe2x80x9cADD/DROPxe2x80x9d function. In general, it is desired to ADD or DROP at one time a set of signals are respective different wavelengths and it is, thus, convenient if the signals to be added, and signals which have been dropped, can be handled in multiplexed form.
In the following references will be made only to WDM and WDM systems. However, unless the context demands otherwise, it is to be understood that these references cover DWDM and DWDM systems.
Implementation of the drop function involves the demultiplexing of the WDM optical signal propagating on the main fiber or waveguide, so as to separate out the components at different respective wavelengths. One or more particular wavelengths which are to be extracted (xe2x80x9cdroppedxe2x80x9d) are selected and routed to a special output channel (the drop channel) different from the main output. The other wavelengths are multiplexed back together and routed to the main output so as to continue propagating along the main fiber or waveguide. It should be recalled that the xe2x80x9cdroppedxe2x80x9d signal is not simply extinguished: it is separated from the other signals so as to follow a different route.
In a similar way, implementation of the add function generally involves the demultiplexing of the WDM signal propagating on the main fiber or waveguide and the addition, to the separated components at different respective wavelengths, of one or more further signals (the xe2x80x9caddedxe2x80x9d signals) at respective individual wavelengths. The original components are then multiplexed back together, along with the added components.
The add/drop functions are generally implemented together in a common device, generally referred to as an xe2x80x9cadd/drop multiplexerxe2x80x9d. Often, a first signal S11 at a wavelength xcexi is dropped and, in the same device, a second signal S12 at this same frequency xcexi is added: such devices are often referred to as xe2x80x9ccross-connectsxe2x80x9d. In the following, the expression xe2x80x9ccross-connectxe2x80x9d will be used to designate devices which implement add/drop functions regardless of the particular wavelengths being dropped and/or added and irrespective of whether both or only one of the functions of adding and dropping is implemented.
One example 10 of a cross-connect is shown schematically in FIG. 1. The cross-connect 10 of FIG. 1 is connected to four optical channels (labeled xe2x80x9cInputxe2x80x9d, xe2x80x9cAddxe2x80x9d, xe2x80x9cOutputxe2x80x9d and xe2x80x9cDropxe2x80x9d in FIG. 1), which are all here assumed to carry WDM signals comprising a plurality (in this simplified example, two) of components at respective different wavelengths. The device 10 comprises two demultiplexers, DEMUX1 and DEMUX2, two multiplexers, MUX1 and MUX2, and an array of optical switches each having first and second input terminals and first and second output terminals (in the simplified example discussed here, the array includes only two switches SW1 and SW2).
In the example illustrated in FIG. 1, the signal propagating in the Input channel is a WDM signal containing component signals at respective wavelengths xcex1 and xcexc2. This signal is applied to the demultiplexer DEMUX1, which separates out the component signals at the different wavelengths and feeds them to first input terminals of respective optical switches SW1 and SW2 of the switch array. The signal propagating in the Add channel is a WDM signal which may contain component signals at any or all of the wavelengths handled by the device (here xcex1 and xcex2). The Add channel signal is applied to demultiplexer, DEMUX2, which separates out the component signals at the different wavelengths and feeds them to respective second input terminals of the optical switches SW1 and SW2.
The first output terminal of each switch in the switch array is connected to a respective input terminal of the multiplexer MUX1, whereas the second output terminal of each switch in the switch array is connected to a respective input terminal of the multiplexer MUX2. The multiplexer MUX1 performs wavelength division multiplexing of the signals applied to its input terminals and outputs the resultant WDM signal to the Output channel. The multiplexer MUX2 performs wavelengths division multiplexing of the signals applied to its input terminals and outputs the resultant WDM signal to the Drop channel.
In the cross-connect 10 of FIG. 1, the optical switches SW1 and SW2 are controlled so as to pass to the multiplexer MUX2 those components of the original input signals which are to be xe2x80x9cdroppedxe2x80x9d, the remainder are passed to the multiplexer MUX1. Similarly, the optical switches SW1 and SW2 are controlled so as to pass to the multiplexer MUX1 all of the components of the Add signals. Thus, by suitably controlling the switches of the switch array it is possible selectively to route individual component signals at different wavelengths present in the WDM signal of the Input channel, and to add new signals at selected wavelengths provided via the Add channel.
The cross-connect 10 of FIG. 1 presents a number of disadvantages. In particular, the design involves use of a relatively large number of components, and the multiplexers and demultiplexers need to be very accurately tuned to the same set of wavelengths.
Alternative prior art cross-connects have been proposed in order to overcome the above-mentioned disadvantages. Specifically, devices have been proposed employing a loop-back configuration so as to enable a single component (a planar optical phased array or xe2x80x9cphasarxe2x80x9d, also known as an arrayed-waveguide grating (AWG)) to perform all of the required multiplexing and demultiplexing.
The arrayed-waveguide grating or phasar was first proposed by M. K. Smit in 1988 in the paper xe2x80x9cNew focusing and dispersive planar component based on an optical phased arrayxe2x80x9d in Electronics Letters, 24, 385. The structure and function of an arrayed-waveguide grating (AWG) multiplexer/demultiplexer will now be briefly summarized with reference to FIGS. 2a and 2b, which illustrate demultiplexer and multiplexer configurations, respectively.
As shown in FIGS. 2a and 2b, the AWG multiplexer/demultiplexer is made up of a phased array of waveguides 1 which are formed in a transparent medium, such as silica, which has been deposited on a substrate (not shownxe2x80x94typically made of silica or silicon) so that there is a constant increment xcex41 in path length from the first waveguide in the array to the second, and so on, through to the last waveguide in the array. The input ends of the waveguides 1 lie along a curve L1, and the output ends of the waveguides 1 lie along a curve L2. Slab waveguides 2a and 2b are formed at respective ends of the array of channel waveguides 1, along the curves L1 and L2. Input and output waveguides 3a, 3b are formed so as to feed signals into and receive signals from the ends of the slab waveguides 2a, 2b remote from the phased array. In general, the phased array of waveguides 1, slab waveguides 2a, 2b, and input and output waveguides 3a, 3b are formed integrally by common deposition and etching processes.
When the arrayed waveguide grating device is used in demultiplexer configuration (FIG. 2a), WDM optical signals are input to a single input waveguide 3a and spread out within the slab waveguide 2a so as to enter each of the waveguides of the array 1. Because the arrayed waveguides 1 have different path lengths, a phase difference is created between the signals reaching the ends of the waveguides 1. (This is comparable to the case where optical signals are incident on a diffraction grating at an oblique angle, xcex8). When these signals exit from the arrayed channel waveguides, they interfere with one another as they propagate within the output slab waveguide 2b.
For a given order of diffraction, m, the component signals at respective different wavelengths present within the original WDM signal will be spatially spread out into a spectrum. These components signals are, thus, demultiplexed and they exit from the slab waveguide 2b at slightly differing spatial positions. These component signals are extracted from the slab waveguide 2b by a plurality of respective output waveguides 3b. It can be considered that the slab waveguide 2b has the effect of focusing component signals at respective wavelengths onto the different respective output waveguides 3b.
The precise spatial location at which a demultiplexed component signal at a given wavelength xcex1 will exit from the slab wavelength 2b, depends upon the length of the slab waveguide 2b and the increment xcex41 in path length between the channel waveguides making up the phased array 1. However, each arrayed waveguide grating is designed to handle a specific set of wavelengths. Thus, the output waveguides 3b are positioned at the spatial locations where it is known (from calculation and design) that the signals at each respective wavelength will exit from the slab waveguide 2b. 
When the arrayed waveguide grating is used in multiplexer configuration (FIG. 2b), the operation is a mirror image of the demultiplexer operation. A plurality of signals at respective different wavelengths is input to slab waveguide 2a via a plurality of input waveguides 3a. These optical signals spread out within slab waveguide 2a and are incident on the input ends of the channel waveguides of the phased array 1. An optical path difference (and corresponding phase difference) builds up between the signals propagating in the different channel waveguides of the phased array 1. When these signals pass into the output slab waveguide 2b and propagate therein, they recombine to form a single, WDM signal. This multiplexed signal is extracted by the single output waveguide 3b. 
Arrayed waveguide gratings may advantageously be used for multiplexing and demultiplexing signals having a very small difference in wavelength. Thus, they are particularly well suited to DWDM applications. The demultiplexer design will, thus, focus on achieving a good spatial separation between the component signals at different but closely-spaced wavelengths which are present in a spectrum at a single order of diffraction, m.
Cross-connects have been proposed making use of AWGs (or xe2x80x9cphasarsxe2x80x9d) and embodying a loop-back configuration have been described in the papers xe2x80x9cArrayed-waveguide grating add-drop multiplexer with loop-back optical pathsxe2x80x9d by Tachikawa et al., Electronics Letters, 25th November 1993, Vol.29, No.24, pp. 2133-2134, and xe2x80x9cFirst InP-based reconfigurable integrated add-drop multiplexerxe2x80x9d by Vreeburg et al., IEEE Photonics Technology Letters. Vol.9, No.2, February 1997, pp.188-190. FIG. 3 is a simplified diagram illustrating the principle involved in such devices.
The cross-connect device 20 of FIG. 3 comprises a phasar 21, advantageously implemented as a planar device using integrated optics technology, and an array of optical switches each having first and second input terminals and first and second output terminals (in the simplified example discussed here, the array includes only two switches SW1 and SW2).
The cross-connect device 20 of FIG. 3 is connected to a Main Input channel and a Main Output channel, both of which carry WDM optical signals. The device 20 is also connected to a plurality of Add signal lines and Drop signal lines each of which can carry a signal at a respective individual wavelength.
More particularly, the waveguide 3a constituting the Main Input channel serves as a first input terminal of the phasar (which can be designated a demultiplexing input terminal) and the waveguide constituting the Main Output channel serves as a first output terminal of the phasar (which can be designated a multiplexing output terminal). There is a set of second output terminals 3b of the phasar 21 (which can be designated demultiplexing output terminals) each of which is connected to a first input terminal of a respective switch in the switch array. There is a set of second input terminals 3axe2x80x2 (which can be designated multiplexing input terminals) of the phasar 21 each of which is connected to a first output terminal of a respective switch in the switch array.
In the example illustrated in FIG. 3, the signal propagating in the Main Input channel is a WDM signal containing component signals at respective wavelengths xcex1 and xcex2. This signal is applied to the demultiplexing input terminal of the phasar 21 and propagates in the input slab waveguide (not shown) so as to enter and propagate in the waveguides of the phased array (also not shown). The component signals at different wavelengths are, in effect, diffracted in the phasar 21 and output at respective ones of the demultiplexing output terminals 3bxe2x80x2 thereof. The separated component signals are then applied to first input terminals of the respective optical switches SW1 and SW2 of the switch array.
Each optical switch may receive at the second input terminal thereof an Add signal at a respective individual wavelength. The first output terminal of each switch in the switch array is connected to a respective one of the multiplexing input terminals 3axe2x80x2 of the phasar 21, whereas the second output terminal of each switch in the switch array is connected to a Drop signal line. The phasar 21 performs wavelength division multiplexing of the signals applied to the multiplexing input terminals thereof and outputs the resultant WDM signal at its multiplexing output terminal 3b. This WDM signal then propagates in the Main Output channel waveguide serving as the phasar multiplexing output terminal 3b. 
In the device 20 of FIG. 3, the optical switches SW1 and SW2 are controlled so as to pass to the respective Drop signal lines those components of the original input signal which are to be xe2x80x9cdroppedxe2x80x9d, the remainder are passed back to the phasar 21. Similarly, the optical switches SW1 and SW2 are controlled so as to pass to the phased array 21 all of the signals on the Add signal lines. Thus, by suitably controlling the switches of the switch array it is possible selectively to route individual component signals at different wavelengths present in the WDM signal of the Main Input channel, and to add new signals at selected wavelengths provided via the Add signal lines.
The cross-connect device 20 of FIG. 3 presents a number of disadvantages. In particular, the added signals and dropped signals are handled individually, at the corresponding switch, rather than in multiplexed form. Moreover, there can be cross-talk between the signals present at the demultiplexer outputs 3bxe2x80x2 of the phased array 21 and the multiplexer output 3b thereof (which serves as the Main Output channel). It has been realized that this cross talk arises because the same order of diffraction is used both by the demultiplexer outputs and the multiplexer output of the phasar 21.
The present invention seeks to overcome the above-mentioned disadvantages of prior art techniques for selective routing of components of a WDM signal. More particularly, the preferred embodiments of the present invention provide devices for selective routing of components signals of WDM optical signals, at respective different wavelengths, using a common element for performing multiplexing and demultiplexing, achieving low crosstalk and accommodating multiplexed add and drop channels.
In various embodiments of selective routing device according to the invention, a common elements is used, in a loop-back configuration, to perform multiplexing and demultiplexing of components of a WDM optical signal having respective different wavelengths, and the multiplexing and demultiplexing functions do not make use of the same order to diffraction.
More particularly, the present invention provides a selective routing device adapted to operate on a wavelength division multiplexed (WDM) optical signal, said WDM optical signal comprising a plurality of component signals at respective different wavelengths, the selective routing device being adapted to route said component signals selectively and comprising: diffraction means adapted to receive optical signals and produce therefrom diffracted optical signals corresponding to a plurality of orders of diffraction including an mth order of diffraction, diffracted optical signals of consecutive orders of diffraction being spaced apart from each other by a known spacing (D); first input means for making said WDM optical signal incident on said diffraction means whereby to cause production of a first diffracted optical signal, corresponding to an mth order of diffraction, said first diffraction optical signal comprising a plurality of first component signals at respective different wavelengths and at respective first locations, adjacent first locations being spatially separated from one another by a known separation (a); selection means for selecting, from among said plurality of first component signals, one or more second component signals at respective different wavelengths, the remaining first component signal(s) constituting third component signal(s); second input means for making said one or more second component signals incident on said diffraction means, said second input means being adapted to present said one or more second component signals to said diffraction means at respective second locations, said second locations being spatially separated from one another and offset from said first locations, said second locations being selected to cause production of a second diffracted optical signal comprising said one or more second component signals wavelength division multiplexed together; and a main output for outputting said second diffracted optical signal; wherein said second diffracted optical signal does not correspond to the mth order of diffraction.
It is possible to implement the invention using a diffraction grating, such as, for example, a ruled bulk diffraction grating), and associated lenses, as the diffraction means and employing first and second input means which are adapted to direct the WDM optical signal and the second component signals, respectively, towards the diffraction grating in directions which are at selected different angles and such that the first and second component signals propagate in opposite senses.
In one embodiment of the invention, the selective routing device comprises a phasar constituted by a phased array of waveguides, a first slab waveguide positioned at one end of the phased array and a second slab waveguide positioned at the other end of the phased array. In this case, the first input means includes a main waveguide arranged to feed signals to the first slab waveguide: the diffraction means is constituted by the phasar: and each of the input means other than the first input means comprises one or more waveguides positioned adjacent the second slab waveguide means. It will be appreciated that the phasar is operated in a bidirectional manner.
The WDM optical signal is applied at the main input of the phasar (via a first one of the slab waveguides) and demultiplexed components thereof are collected (via the second slab waveguide) at first locations corresponding to an mth order of diffraction. Those of the demultiplexed component signals which are to be maintained in the main signal are selected as second component signals and routed back to the phasar at second locations (on the second slab waveguide) so as to be multiplexed together for output (via the first slab waveguide). Those of the demultiplexed component signals which are to be routed in a different manner from the main signal (dropped) are designated third components signals and are not routed back to the second locations.
In one embodiment of the invention, those demultiplexed (second) component signals which are selected to be maintained in the main signal are fed back to the phasar at second locations which correspond to an order of diffraction (m+1 or mxe2x88x921) adjacent to that used for the initial demultiplexing outputs. However, a small degree of crosstalk will arise at these locations due to energy, from the initial WDM optical signal, which is diffracted into the m+1 and mxe2x88x921 orders of diffraction.
Thus, in the preferred embodiments of the invention the second component signals are fed back to the phasar at second locations which are offset both from the mth order of diffraction and the m+1th and the m-xe2x88x921th orders of diffraction. It has been found to be particularly advantageous to use second locations which are offset from the first locations (mth order of diffraction) by a distance substantially equal to the sum of the free spectral range (that is the spacing (D) between consecutive orders of diffraction) and the adjacent channel spacing (that is, the separation (a) between adjacent ones of the first locations).
The third component signals (that is, those of the demultiplexed (first) component signals which are to be routed differently from the main signal) can be directed to a second output channel (Drop channel) on an individual basis by means of separate signal lines together constituting a second output channel.
However, it may be preferable to output the third component signals in multiplexed form. This can be done by making use of third input means for making said third component signal(s) incident on said diffraction means, said third input means being adapted to present said third component signal(s) to said diffraction means at respective third locations, said third locations being spatially separated from one another and offset from said first and second locations, said third locations being selected to cause production of a third diffracted optical signal comprising said third component signal(s) wavelength division multiplexed together; and outputting the third diffracted optical signal to the second output channel.
In preferred embodiments of the invention, the third input means is a further waveguide arranged to feed signals to the phasar slab waveguide that outputs the first diffracted optical signal. Thus, a single phasar performs all of the demultiplexing and multiplexing required for selective routing of components of the input WDM optical signal and output of the dropped components in a multiplexed form.
The third locations can be positioned corresponding to an order of diffraction adjacent the mth order of diffraction. For example, if the first locations corresponds to the mth order of diffraction and the second locations correspond to the m+1th order of diffraction, the third locations can correspond to the mxe2x88x921th order of diffraction. However, once again, there would be a small degree of crosstalk due to energy originating from the initial WDM optical signal.
Thus, it is preferred that said third locations should be offset from positions corresponding to the mth order of diffraction and adjacent orders of diffraction. It has been found to be convenient to employ third locations offset from the mth order of diffraction by a distance substantially equal to the sum of the free spectral range (D) and the adjacent channel spacing (a), and offset from the second locations by a distance (2D+2a) substantially equal to twice said sum.
In certain embodiments of the invention, components at particular wavelengths can be added to the main signal and/or both added to and dropped therefrom. Each such (fourth) component signal to be added can conveniently be included among the second component signals output from the selection means to the second input means. The fourth component signals can be added on an individual basis by means of separate signals lines together constituting a second input channel. However, it is preferable if the added signals are handled in multiplexed form.
Thus, in the preferred embodiments of the invention providing the added function, there is a second input (Add) channel adapted to receive a further wavelength division multiplexed (WDM) optical signal, the further WDM optical signal comprising a plurality of fourth component signals at respective different wavelengths; fourth input means for making said further WDM optical signal incident on said diffraction means at a fourth location, said fourth location being offset from the first locations, whereby to cause production of a fourth diffracted optical signal, said fourth diffracted optical signal comprising said fourth component signal(s) at respective fifth locations, said fifth locations being spatially separated from one another; transfer means for routing said fourth component signal(s) from said fifth locations to said selection means; wherein the selection means is adapted to include said fourth component signals among said one or more second component signals output to said second input means.
In such preferred embodiments of the invention, a single phasar provides all of the demultiplexing and multiplexing required for selective routing of the original components of the WDM optical signal and for adding (and, if desired, dropping) component signals in multiplexed form.
In the selective routing devices according to the invention, the selection means is advantageously implemented as an array of optical switches. These may be optical 2xc3x972 switches.
Preferably, the principal elements of the devices according to the preferred embodiments of the invention are fabricated as planar lightwave devices. Thus, for example, at least the phasar and, optionally, the combination of the phasar and the switch array, will be fabricated by conventional planar lightwave circuit techniques.
It will be seen that certain embodiments of the present invention constitute add/drop multiplexers. These devices have the advantage that they employ a single element for all the required demultiplexing and multiplexingxe2x80x94thus avoiding the need for careful wavelength tuning of different circuit elements. Moreover, by using multiplexing inputs which correspond to an order of diffraction different from that used by the demultiplexing inputs (or which correspond to locations offset from such orders of diffraction), crosstalk in the preferred embodiments is low. Further, multiplexed Add and Drop channels may be accommodated.